Process for producing oxide superconductive thin-film

ABSTRACT

Provided is a method of producing an oxide superconducting film on a single-crystal substrate by depositing, on the single-crystal substrate, substances scattered from a raw material due to irradiation with laser beams according to a pulsed-laser deposition method, wherein the irradiation of the raw material is performed in a manner such that the repetition frequency of the pulse irradiation of the laser beams is divided into at least two steps. Thus, an oxide superconducting film having a high critical current density can be produced by the method.

TECHNICAL FIELD

The present invention relates to methods of producing oxidesuperconducting films, and in particular, relates to methods ofproducing oxide superconducting films on single-crystal substrates bypulsed-laser deposition.

BACKGROUND ART

For example, a method of depositing a superconducting film on asingle-crystal substrate by vapor deposition is described in thefollowing document:

B. Schey, et al., “Large Area Pulsed Laser Deposition of YBCO ThinFilms”, IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, Vol. 9, No. 2,JUNE 1999, pp. 2359-2362.

In the method described in this document, a laser ablation process isused for forming a film. More specifically, an oxide superconductingfilm is formed on a substrate by depositing substances scattered from atarget due to irradiation with a laser beam. The document states that alarge area oxide superconducting film, such as a YBa₂Cu₃O_(7-x)(YBCO)film, can be formed by this method. However, since the conditions of aprocess for yielding a high critical current density were notestablished in the method described in this document, the oxidesuperconducting films formed by the method inevitably had a low criticalcurrent density.

DISCLOSURE OF INVENTION

Accordingly, it is an object of the present invention to provide amethod for producing an oxide superconducting film having a highercritical current density than that of known examples.

The method of the present invention for producing an oxidesuperconducting film is a method in which an oxide superconducting filmis formed on a single-crystal substrate by depositing substancesscattered from a raw material due to irradiation with laser beamsaccording to a pulsed-laser deposition method, and which ischaracterized in that the irradiation of the raw material is performedin a manner such that the repetition frequency of the pulse irradiationof the laser beams (hereinafter referred to as a laser frequency) isdivided into at least two steps. The laser used in the method may be anexcimer laser, for example, ArF laser having a wavelength of 193 nm, KrFlaser having a wavelength of 248 nm, or XeCl laser having a wavelengthof 308 nm.

As a result of extensive investigations, the inventors have found thatan oxide superconducting film produced by at least two steps of laserirradiations at different laser frequencies has a higher criticalcurrent density than an oxide superconducting film formed by one step oflaser irradiation at a single laser frequency.

The process of forming an oxide superconducting film can generally bedivided into two steps: a step of forming a seed crystal on thesubstrate surface and a step of growing the crystal. In the presentinvention, it is possible to divide laser frequencies into at least twosteps: a frequency suitable for forming a seed crystal as a first stepand another frequency suitable for growing the crystal as a second step.Accordingly, an oxide superconducting film having a high criticalcurrent density can be formed.

In the above-mentioned method of producing the oxide superconductingfilm, the first laser frequency is preferably smaller than the secondlaser frequency.

Thus, by controlling the laser frequency as described above, an oxidesuperconducting film having a higher critical current density can beproduced.

In the above-mentioned method of producing the oxide superconductingfilm, the energy per pulse (hereinafter referred to as laser power) ispreferably 400 mJ or more. By setting a laser power at such a level, anoxide superconducting film having a high critical current density can beproduced.

In the above-mentioned method of producing the oxide superconductingfilm, the temperature of a substrate during the pulsed-laser depositionis preferably more than or equal to 600° C. and less than 1,200° C. Anoxide superconducting film having a high critical current density can beproduced by setting the temperature of the substrate to such a level.

In the above-mentioned method of producing the oxide superconductingfilm, the gas pressure during the pulsed-laser deposition is within therange of 1.33 Pa to 100 Pa, and preferably from 1.33 Pa to 66.66 Pa. Bysetting the gas pressure at such a level, an oxide superconducting filmhaving a high critical current density can be produced.

In the above-mentioned method of producing the oxide superconductingfilm, the atmosphere during the pulsed-laser deposition preferablycontains oxygen. With such presence of oxygen, an oxide superconductingfilm having a high critical current density can be produced.

In the above-mentioned method of producing the oxide superconductingfilm, the oxide superconducting film preferably has an RE123composition. The RE is a substance including at least one of arare-earth element and yttrium. The oxide superconducting film having aRE123 composition, which can transport a high current, is suitable foran electric-power application.

The “RE123 composition” in the present specification is represented byRE_(x)Ba_(y)Cu_(z)O_(7-d), where 0.7≦x≦1.3, 1.7≦y≦2.3, and 2.7≦z≦3.3. REin the “RE123 composition” indicates a substance that includes at leastone of a rare-earth element and yttrium.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a method of producing an oxide superconducting filmaccording to an embodiment of the present invention.

FIG. 2 illustrates steps for forming an oxide superconducting film intwo steps at different laser frequencies in a pulsed-laser depositionprocess.

FIG. 3 is a cross-sectional view schematically showing a configurationof an oxide superconducting film according to an embodiment of thepresent invention.

FIG. 4 shows the relationship between the gas pressure duringpulsed-laser deposition and the critical current density of anHoBa₂Cu₃O_(x) (HoBCO) superconducting layer under a self-magnetic fieldin liquid nitrogen.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments according to the present invention will now be describedwith reference to drawings.

FIG. 1 illustrates a method of producing an oxide superconducting filmaccording to an embodiment of the present invention. With reference toFIG. 1, a substrate 10 is disposed on a heater 2 at a predeterminedangle to a target (raw material) 1. With this arrangement, the substrate10 is covered with a mask (not shown) at a predetermined area. Thetarget 1 is irradiated with a laser beam 3 to perform laser ablation. Asubstance (plume) 4 scattered from the target 1 is vapor deposited onthe exposed surface of the substrate 10 to form an oxide superconductingfilm.

The laser frequencies of the laser irradiation to the target 1 aredifferent between the two steps (step S1 and step S2), as shown in FIG.2. The laser frequency of the laser irradiation in the first step (stepS1) is preferably smaller than that in the second step (step S2). Thelaser power of the laser irradiation is preferably at least 400 mJ, morepreferably at least 600 mJ, and most preferably from 800 mJ to 1,000 mJ.

The temperature of the substrate 10 during the pulsed-laser depositionis preferably more than or equal to 600° C. and less than 1,200° C., andmore preferably more than or equal to 800° C. and less than 1,200° C.

The gas pressure during the pulsed-laser deposition is preferably from1.33 Pa to 66.66 Pa, and the atmosphere during the pulsed-laserdeposition preferably contains oxygen.

In the above embodiment, two different laser frequencies are employed inthe two steps, but three or more different laser frequencies may beused.

The oxide superconducting film formed by the above-mentioned method willnow be described.

FIG. 3 schematically shows a cross-sectional view of the configurationof an oxide superconducting film according to an embodiment of thepresent invention. With reference to FIG. 3, the oxide superconductingfilm 13 is formed on the substrate 10. The substrate 10 includes asingle-crystal sapphire substrate 11 and an intermediate layer 12composed of, for example, cerium oxide. The oxide superconducting film13 is composed of, for example, HoBa₂Cu₃O_(x) and has a thickness T.

The oxide superconducting film 13 may be formed on the single-crystalsapphire substrate 11 directly. Examples of the present invention willnow be described.

EXAMPLE 1

A holmium-based superconducting film (HoBa₂Cu₃O_(x):HoBCO) was formed ona single-crystal substrate of lanthanum aluminate by a laser ablationprocess using a XeCl excimer laser having a wavelength of 308 nm. Inthis process, the repetition frequency of the laser irradiation wasvaried as a parameter. The film deposition was conducted at a repetitionfrequency of a first step for ten minutes, and thereafter at arepetition frequency of a second step for ten minutes. The atmospherecontained oxygen gas at 13.33 Pa, the temperature of the substrate was900° C. and the laser power was 900 mJ; these conditions were keptconstant during the film deposition. To determine the characteristics ofthe superconducting layer, the critical current density of the HoBCOsuperconducting layer under a self-magnetic field in liquid nitrogen wasmeasured. The results are shown in Table I. TABLE I Critical currentdensity of HoBCO superconducting layer under self-magnetic field inliquid nitrogen at various repetition frequencies in first and secondsteps. Frequency in Frequency in second step (Hz) first step (Hz) 1 5 2040 100 200 1 0.9 1.9 3.0 2.8 1.5 1.3 5 0.4 0.7 3.0 4.0 3.2 1.8 20 0.30.4 0.8 2.0 1.8 1.5 40 0.2 0.3 0.4 0.8 1.2 1.2 100 0.1 0.1 0.3 0.3 0.61.2 200 0.0 0.1 0.1 0.2 0.3 1.1

As shown in Table I, in the cases where the repetition frequency oflaser irradiation in the first step was smaller than that in the secondstep, the critical current density was high.

EXAMPLE 2

A holmium-based superconducting film (HoBa₂Cu₃O_(x):HoBCO) was formed ona single-crystal substrate of lanthanum aluminate by a laser ablationprocess. In this process, laser energy was varied as a parameter. Thefilm deposition was conducted at a repetition frequency of 5 Hz for tenminutes in the first step, and thereafter at a repetition frequency of40 Hz for ten minutes in the second step. The atmosphere containedoxygen gas at 13.3 Pa and the temperature of the substrate was 900° C.;these conditions were kept constant during the film deposition. Todetermine the characteristics of the superconducting layer, the criticalcurrent density of the HoBCO superconducting layer under a self-magneticfield in liquid nitrogen was measured. The results are shown in TableII. TABLE II Critical current density of HoBCO superconducting layerunder self- magnetic field in liquid nitrogen at various laser powerlevels. Laser power (mJ) 200 300 400 500 600 700 800 900 1,000 Critical0.2 0.3 1.5 2.3 2.8 3.4 3.6 4.0 3.8 current density of HoBCO super- con-ducting layer (MA/ cm²)

As shown in Table II, in the ceases where the laser energy was 400 mJ ormore, the critical current density was high.

EXAMPLE 3

A holmium-based superconducting film (HoBa₂Cu₃O_(x):HoBCO) was formed ona single-crystal substrate of lanthanum aluminate by a laser ablationprocess. In this process, the temperature of the substrate was varied asa parameter. The film deposition was conducted at a repetition frequencyof 5 Hz for ten minutes in the first step, and thereafter at arepetition frequency of 40 Hz for ten minutes in the second step. Theatmosphere contained oxygen gas at 13.3 Pa and the laser power was 900mJ; these conditions were kept constant during the film deposition. Todetermine the characteristics of the superconducting layer, the criticalcurrent density of the HoBCO superconducting layer under a self-magneticfield in liquid nitrogen was measured. The results are shown in TableIII. TABLE III Critical current density of superconducting layer underself-magnetic field in liquid nitrogen at various substratetemperatures. Substrate temperature (° C.) 400 500 600 800 900 1,0001,100 1,200 Critical current 0.1 0.3 1.8 3.1 4.0 3.8 3.5 0.3 density ofHoBCO (MA/cm²)

As shown in Table III, in the cases where the temperature of thesubstrate was more than or equal to 600° C. and less than 1,200° C., thecritical current density was high.

EXAMPLE 4

A holmium-based superconducting film (HoBa₂Cu₃O_(x):HoBCO) was formed ona single-crystal substrate of lanthanum aluminate by a laser ablationprocess. In this process, oxygen-gas pressure during the film depositionwas varied as a parameter. The film deposition was conducted at arepetition frequency of 5 Hz for ten minutes in the first step, andthereafter at a repetition frequency of 40 Hz for ten minutes in thesecond step. The temperature of the substrate was 900° C. and the laserpower was 900 mJ; these conditions were kept constant during the filmdeposition. To determine the characteristics of the superconductinglayer, the critical current density of the HoBCO superconducting layerunder a self-magnetic field in liquid nitrogen was measured. The resultsare shown in Table IV and FIG. 4. TABLE IV Critical current density ofHoBCO superconducting layer under self- magnetic field in liquidnitrogen at various gas pressures. Gas pressure (Pa) 0.07 0.13 1.33 6.6713.33 26.66 66.66 100 133.3 Critical 0.1 0.3 1.2 2.7 4.0 2.8 2.2 1.1 0.2current density of HoBCO (MA/ cm²)

As shown in Table IV and FIG. 4, the critical current density was highin the cases where the gas pressure was within a range of 1.33 Pa to 100Pa, and especially within the range of 1.33 Pa to 66.66 Pa.

EXAMPLE 5

A holmium-based superconducting film (HoBa₂Cu₃O_(x):HoBCO) was formed ona single-crystal substrate of lanthanum aluminate by a laser ablationprocess. In this process, the type of atmospheric gas during the filmdeposition was varied as a parameter. The film deposition was conductedat a repetition frequency of 5 Hz for ten minutes in the first step, andthereafter at a repetition frequency of 40 Hz for ten minutes in thesecond step. The temperature of the substrate was 900° C., the laserpower was 900 mJ and the atmospheric gas pressure was 13.33 Pa; theseconditions were kept constant during the film deposition. To determinethe characteristics of the superconducting layer, the critical currentdensity of the HoBCO superconducting layer under a self-magnetic fieldin liquid nitrogen was measured. The results are shown in Table V. TABLEV Critical current density of HoBCO superconducting layer under self-magnetic field in liquid nitrogen in various types of gas. Type of gasNitrogen Dinitrogen Argon Oxygen Hydrogen dioxide monoxide Criticalcurrent 0.1 4.0 0.2 0.3 0.3 density of HoBCO (MA/cm²)

As shown in Table V, when the gas was oxygen, the critical currentdensity was high.

The embodiments and examples are intended merely as exemplifications ofthe present invention in all aspects and do not limit the scope of thepresent invention. The scope of the present invention is defined by theclaims not by the above-mentioned descriptions. The scope of the presentinvention encompasses every alternative equivalent to or within thescope of the claims.

Industrial Applicability

As described above, a method of producing an oxide superconducting filmof the present invention includes at least two steps, in which a firstlaser frequency that is suitable for a seed-crystal-forming process anda second laser frequency that is suitable for a crystal-growing processare used. This results in forming an oxide superconducting film having ahigh critical current density.

1. A method of producing an oxide superconducting film on asingle-crystal substrate by depositing, on the single-crystal substrate,substances scattered from a raw material due to irradiation with laserbeams according to a pulsed-laser deposition method, wherein theirradiation of the raw material is performed in a manner such that therepetition frequency of the pulse irradiation of the laser beams isdivided into at least two steps.
 2. A method of producing an oxidesuperconducting film according to claim 1, wherein the laser frequencyof a first step is smaller than the laser frequency of a second step. 3.A method of producing an oxide superconducting film according to claim1, wherein the laser power is 400 mJ or more.
 4. A method of producingan oxide superconducting film according to claim 1, wherein thetemperature of the single-crystal substrate during the pulsed-laserdeposition is more than or equal to 600° C. and less than 1,200° C.
 5. Amethod of producing an oxide superconducting film according to claim 3,wherein the temperature of the single-crystal substrate during thepulsed-laser deposition is more than or equal to 600° C. and less than1,200° C.
 6. A method of producing an oxide superconducting filmaccording to claim 1, wherein the gas pressure during the pulsed-laserdeposition is within the range of 1.33 Pa to 66.66 Pa.
 7. A method ofproducing an oxide superconducting film according to claim 3, whereinthe gas pressure during the pulsed-laser deposition is within the rangeof 1.33 Pa to 100 Pa.
 8. A method of producing an oxide superconductingfilm according to claim 4, wherein the gas pressure during thepulsed-laser deposition is within the range of 1.33 Pa to 100 Pa.
 9. Amethod of producing an oxide superconducting film according to claim 1,wherein the gas pressure during the pulsed-laser deposition is withinthe range of 1.33 Pa to 66.66 Pa.
 10. A method of producing an oxidesuperconducting film according to claim 3, wherein the gas pressureduring the pulsed-laser deposition is within the range of 1.33 Pa to66.66 Pa.
 11. A method of producing an oxide superconducting filmaccording to claim 4, wherein the gas pressure during the pulsed-laserdeposition is within the range of 1.33 Pa to 66.66 Pa.
 12. A method ofproducing an oxide superconducting film according to claim 1, whereinthe atmosphere during the pulsed-laser deposition contains oxygen.
 13. Amethod of producing an oxide superconducting film according to claim 3,wherein the atmosphere during the pulsed-laser deposition containsoxygen.
 14. A method of producing an oxide superconducting filmaccording to claim 4, wherein the atmosphere during the pulsed-laserdeposition contains oxygen.
 15. A method of producing an oxidesuperconducting film according to claim 6, wherein the atmosphere duringthe pulsed-laser deposition contains oxygen.
 16. A method of producingan oxide superconducting film according to claim 1, wherein the oxidesuperconducting film comprises an RE123 composition, where RE iscomposed of at least one of a rare-earth element and yttrium.
 17. Amethod of producing an oxide superconducting film according to claim 3,wherein the oxide superconducting film comprises an RE123 composition,where RE is composed of at least one of a rare-earth element andyttrium.
 18. A method of producing an oxide superconducting filmaccording to claim 4, wherein the oxide superconducting film comprisesan RE123 composition, where RE is composed of at least one of arare-earth element and yttrium.
 19. A method of producing an oxidesuperconducting film according to claim 6, wherein the oxidesuperconducting film comprises an RE123 composition, where RE iscomposed of at least one of a rare-earth element and yttrium.
 20. Amethod of producing an oxide superconducting film according to claim 12,wherein the oxide superconducting film comprises an RE123 composition,where RE is composed of at least one of a rare-earth element andyttrium.